Nitric Acid-Assisted Sol–Gel Synthesis of Defect-Rich TiO2 Nanoparticles: Structural Evolution and Visible-Light Photocatalytic Performance

TiO2 nanoparticles were synthesized by a nitric acid-assisted sol–gel route using three different amounts of nitric acid (NA) (0, 0.05, and 0.10 mL HNO3) to investigate how controlled acid addition influences their structural, optical, and photocatalytic behavior under visible-light irradiation. X-ray diffraction and Raman spectroscopy confirmed the formation of phase-pure anatase TiO2, with slightly increased crystallinity and crystallite size upon NA incorporation. UV–Vis absorption and Tauc analysis revealed a systematic blue shift in the absorption edge accompanied by band-gap widening, attributed to electron–hole confinement and defect-state modification. Photoluminescence spectra showed enhanced visible emission with increasing acid content, indicating a higher density of oxygen vacancies and Ti3+ centers. SEM–EDX analysis verified homogeneous morphology, Ti–O stoichiometry, and the absence of extrinsic impurities. Although the TiO2 sample prepared with 0.10 mL of HNO3 (FNA) showed a wider band gap and slightly larger crystallite size, it still delivered the highest photocatalytic performance in methylene blue degradation, reaching about 74.8% removal after 240 min of visible-light exposure. This unexpected behavior can be explained by a defect-related mechanism in which NA promotes the formation of surface oxygen vacancies and Ti3+ sites. Because of these defects, new electronic states appear between the valence and conduction bands, allowing the material to absorb lower-energy light and improving how electrons interact with the dye.